Ultrasonic imaging systems for producing real-time images of internal portions of the human body are well-known. In one such system, an array of ultrasonic transducers placed in contact with the body converts short electrical pulses into corresponding pressure waves. The electrical pulses can be applied to each individual transducer in the array and by choosing the application time of the pulses to each transducer relative to the other transducers in the array, the pressure waves generated by each transducer can be formed into a "transmit beam" which propagates in a predetermined direction from the array.
As the pressure waves in the transmit beam pass through the body, a portion of the acoustic energy is reflected back towards the transducer array whenever the waves encounter tissues having different acoustic characteristics. An array of receiving transducers (which may be the same as the transmitting array) is provided for converting the reflected pressure pulses into corresponding electrical pulses. The reflected pressure pulses are received by each transducer in the receiving array and by suitably choosing relative delays between the signals generated by each transducer and combining the signals, the received pressure waves located in a "receiving beam" can be emphasized preferentially to other pressure pulses. As with the transmit beam, the relative transducer delays can be adjusted so that the receiving beam extends in any desired direction from the transducer array.
It is also possible to "focus" the received acoustic signals at a point along the receiving beam. This is done by selectively adjusting relative signal delays between the transducers so that the electrical signals generated by the receiving transducers are superimposed in time for signals received from a point along the receiving beam at a predetermined distance from the transducer array, but are not superimposed for other signals. Consequently, when the signals are combined, a strong signal is produced from signals corresponding to this point whereas signals arriving from other points at different times have random phase relationships and therefore destructively interfere.
A two-dimensional image plot or sector image can be generated with this system by adjusting the acoustic transducers to generate or "shoot" a transmit beam at a selected angular direction from the transducer array. The receiving transducers are then adjusted to generate a receiving beam at the same angle as the transmitting beam. The receiving transducers are adjusted to focus the receiving beam at sequentially increasing distances from the transducer array along the predetermined transmit beam angle. The received signals for each sequential focal point are stored. The transmit and receive beams are then moved by a predetermined angular amount and the process of acquiring signals is repeated. The started signals are then processed to generate a wedge-shaped acoustic image called a sector.
Since the distances between any desired focal point along the receiving beam and the various receiving transducers are different, the reflected pressure pulses arrive at the transducers at different times, thereby generating electrical signals at different times. It is therefore necessary to introduce compensating electrical delays between each transducer and the signal summing point so that the time of arrival of all of the electrical signals at the summing point is the same regardless of which transducer is involved. The collection of transducer compensating delays and the signal summing circuitry is normally referred to as a "beamformer" and is described, for example, in U.S. Pat. No. 4,140,022 issued to the assignee of the present invention. The description of the beamformer apparatus described therein is hereby incorporated by reference.
The output of the beamformer is generally a radio-frequency signal representing the amplitude of the received pressure pulses. The signals are often a function of the angle (.theta.) of the receive beam and the radial distance (R) along the receive beam at which the focal point occurs. Consequently, the signals are said to be in R-.theta. coordinates. It is also possible, using conventional construction methods, to construct a beamformer which generates scanning information in other coordinate systems, such as a linear scan. However, by considering small, localized areas, signals expressed in these other coordinate systems can be converted to R-.theta. coordinates. Therefore, the following discussion will assume R-.theta. coordinates without loss of generality.
Generally, the signals are displayed on a display monitor such as a television or raster-scan monitor and, thus, the format of the signals must be converted from R-.theta. coordinates to the X-Y coordinates used in the television display. This conversion is performed by a device called an X-Y scan converter. Since actual data is available in R-.theta. coordinates at discrete angular positions, the scan converter must generate the required X-Y values by interpolating between the R-.theta. coordinate values. The construction and operation of such scan converters is well-known. For example, scan converters are discussed in detail in U.S. Pat. Nos. 4,468,747 and 4,471,449, both assigned to the assignee of the present invention. The description of these patents is hereby incorporated by reference and, accordingly, the detailed construction of scan converters will not be discussed further herein.
It has been found that with some conventional scan converter systems certain problems occur. One such problem is that the images produced by the system often have "artifacts" in the reconstructed image. Artifacts are visual anomalies that appear in the displayed image but are not present on the actual object. Such anomalies may consist of radiating lines, checkerboard patterns or speckles and are generally related to imperfect reconstruction of the image.
Another problem with prior art systems is that they often have limited resolution. One known method of increasing image resolution is to increase the number of acoustic lines which are shot by reducing the angular increment between lines. Obviously, such an approach increases the overall time necessary to obtain the acoustic data and reconstruct the image. Since many ultrasonic imaging systems are used for imaging moving objects such as heart valves, it is of prime importance to generate an image as fast as possible (by increasing the "frame rate" or the number of images generated per unit time) so that the object motion can be depicted as accurately as possible. The frame rate can be increased by decreasing the number of lines which are shot to produce each image. However, as previously discussed, this also reduces the overall resolution of the image. Consequently, in prior art systems there has been a trade-off between resolution and frame rate.
Accordingly, it is an object of the present invention to provide a method and apparatus for increasing resolution without correspondingly reducing the frame rate of the system.
It is a further object of the present invention to increase the signal-to-noise ratio of the system or increase the frame rate without correspondingly increasing the amount of circuitry or time involved generating an acoustic image.
It is a further object of the present invention to utilize additional information normally discarded during the prior art reconstruction process to provide better resolution upon reconstruction.
It is a further object of the present invention to reduce artifacts in the acoustic image produced by imperfect prior art reconstruction processes.
It is still a further object of the present invention to increase the resolution without increasing the acoustic line density.